A conventional gantry-type stage structure is provided with a beam disposed above and extending across a stationary base for an optical processing device to be mounted thereon. A beam as disclosed in Taiwanese Patent No. M401496 includes an elongated rectangular cuboid hollow body elongated in a left-and-right direction and defining an accommodating space therein, and a plurality of support plates disposed erectly in the accommodating space and arranged to be spaced apart from one another in the left-and-right direction. The hollow body has a housing wall which is formed with numerous screw holes therein such that an optical processing device such as an optical detecting or repair device is securely mounted on the housing wall for being moved therewith. Many support plates are required to be disposed compactly so as to provide the beam with a sufficient structural strength, which renders the beam weight heavy and hence adversely affects the processing operation of the beam. Additionally, it is needed to renew the whole body of the beam once some of the screw holes are damaged, causing the maintenance to be inconvenient.